Urea and ammonium nitrate plants are two commercially important examples of synthesis plants of the kind in which the synthesis reaction reaches an equilibrium rather than going to completion, in which the product is relatively less volatile than the starting materials, and in which the raw product is processed though evaporator and solidification stages to produce a final product. The objects of these conventional after-treatment steps are to produce a relatively high purity product, recover solvent and unreacted starting materials, and reduce pollution in effluent streams.
In urea plants the vaporous overhead stream from the evaporator stage or stages contains significant quantities of entrained urea which, unless removed, ends up in the condensate formed from the overhead stream. Its presence there is troublesome, because it must be hydrolyzed (converted to NH.sub.3 and CO.sub.2). The hydrolysis products are separated from the water in the condensate and sent, along with unreacted starting material separated from the condensate, back into the reactor for reconversion into urea. These steps are necessary in order to raise the condensate to sufficient purity for discharge as effluent, as well as for efficient use of materials.
Similarly, in ammonium nitrate plants, the vaporous overhead stream from the evaporator stage or srtages contains entrained ammonium nitrate, which must be removed from the condensate before it can be discharged.
In conventional urea plants, the level of entrained urea in the evaporator overhead streams is reduced through the use of centrifugal or impingement type vane separators or similarly acting devices, but these devices still leave an objectionably high urea concentration in the condensate. This increases both plant capital and operating costs, because of the large condensate treatment system which must be provided and operated. In addition, it introduces inefficiencies and decreases plant capacity, because the already formed urea in the condensate is hydrolyzed back into starting materials which are then resynthesized into urea again. A portion of the synthesis section capacity is thus dedicated to this looping of materials, and is unavailable for production of usable product.